Mobile devices incorporate an audio amplifier to allow a user to listen to a conversation or enjoy a musical program or audio book. Many mobile devices use a class-D audio amplifier as part of the audio circuit. This class-D audio amplifier incorporates two complementary metal-oxide silicon (CMOS) transistors that are biased on or off These CMOS transistors generate a digital output which is about ten times the highest frequency of the speaker. The speaker acts as a low pass filter in the audio circuit. The pulse modulation out of the class-D amplifier is the digital component and is filtered out by the speaker.
Most mobile devices incorporate a power supply to power the device and also to provide power for the audio amplifier circuit. The power supply generates noise in the course of generating power and this noise resides inside the low pass filter, or speaker passband. As a result the listener hears this noise through the speaker. This noise is a low pass filtered analog output.
The class-D switching amplifier suffers from poor power supply rejection ration (PSRR). This means that the noise present at the speaker is more likely to occur. PSRR is typically limited by matching the feedback resistors. However, improving matching of the feedback resistors requires increasing the area, which may not be possible on the die, as many functions must be provided and the area for matching may be less than needed to provide the best sound quality.
A further challenge to improving PSRR and providing better sound quality is that the matching required and hence the specific resistors needed varies with the process being studied. There is a need in the art for a compact circuit for cancelling power supply noise for a class-D audio amplifier utilizing a multiple feedback loop.